1. Technical Field
The present invention relates to a mobile station and a mobile communication method.
2. Background Art
In Release. 8 of the 3GPP, in the GSM (Global System for Mobile Communications) scheme, the UMTS (Universal Mobile Telecommunications System) scheme, or the LTE (Long Term Evolution) scheme, it is considered to apply “priority control”.
In the priority control, for a purpose of dispersing a network load, a mobile station UE performs a cell selection process (Cell Reselection) based on a priority for a frequency (carrier) or a group of frequencies of each radio access network.
For example, in such priority control, “0” to “7” can be set as the above-described priority, where “7” is the highest priority, and the mobile station UE, as a camp-on cell, is configured to preferentially select a cell with a higher priority when a radio quality (RSCP or RSRP) satisfies a predetermined condition.
It is noted that the mobile station UE may acquire the priority by broadcast information in each cell, and may acquire the priority by dedicated signaling (“RRC Connection Release message” at the time of releasing an RRC connection, for example) for each mobile station UE.
In a radio access network of the existing UMTS scheme or GSM scheme, a disposition of each cell, a parameter for a cell selection process, etc., are set in order to optimize a cell selection process based on a radio quality (e.g., RSCP or Ec/N0). Therefore, as an operator, it is more desirable to perform the cell selection process that has been performed in the existing network rather than the new priority control.
That is, in the radio access network of the UMTS scheme or the radio access network of the GSM scheme, as the operation of the mobile station UE, the operator wishes to implement a cell re-selection process based on the conventional radio quality for a cell selection process (Intra-Frequency Cell Reselection) among a plurality of cells that use an identical frequency or a cell selection process (Inter-Frequency Cell Reselection) among a plurality of cells that use a different frequency; and wishes to apply the priority control for a cell selection process (Inter-RAT Cell Selection) between a cell within the radio access network of the UMTS scheme and a cell within the radio access network of the LTE scheme.
In this case, in the Release.8 of the 3GPP, in order to realize the above-described operation, in the GSM scheme or the UMTS scheme (Legacy System), when the priority for a frequency (carrier) or a group of frequencies of each radio access network is not set, it is agreed that the cell re-selection process is implemented based on the conventional radio quality.
One example of a mobile communication system for realizing the above-described operation will be explained with reference to FIG. 4 and FIG. 5.
As illustrated in FIG. 4, in the mobile communication system, a cell LTE-F0 within a radio access network of an of the LTE scheme, a cell UTRA-F1 within a radio access network of the UMTS scheme, and a cell UTRA-F2 within a radio access network of the UMTS scheme are overlaid.
In this case, the frequency used in the cell LTE-F0 is “F0”, the frequency used in the cell UTRA-F1 is “F1”, and the frequency used in the cell UTRA-F2 is “F2”.
The broadcast information (SIB3/SIB6) in the cell LTE-F0 includes “5” as the priority of the cell LTE-F0, includes “3” as the priority of the cell UTRA-F1, and includes “3” as the priority of the cell UTRA-F2.
Further, the broadcast information (SIB19) in the cell UTRA-F1 includes “5” as the priority of the cell LTE-F0, and includes “3” as the priority of the cell UTRA-F1.
In this case, the reason why the priority of the cell UTRA-F2 is not set to the broadcast information in the cell UTRA-F1 is because a cell re-selection process based on the conventional radio quality is applied to a cell selection process between the cell UTRA-F1 and the cell UTRA-F2.
According to the present 3GPP specification, if the mobile station UE does not have any priority for a frequency different from a frequency (carrier) used in a cell within a radio access network of the UMTS scheme when the mobile station UE resides in a cell within a radio access network of the UMTS scheme, then the mobile station UE usually performs the cell re-selection process based on the conventional radio quality.
Therefore, in such an example, at the time of residing in the cell UTRA-F1, the mobile station UE does not have the priority for a frequency (frequency used in the cell UTRA-F2, for example) different from the frequency (carrier) used in the cell, in the radio access network of a UMTS scheme, and as a result, the mobile station UE performs the cell re-selection process based on the conventional radio quality.
Similarly, the broadcast information (SIB19) in the cell UTRA-F2 includes “5” as the priority of the cell LTE-F0, and includes “3” as the priority of the UTRA-F2.
As described above, in the broadcast information in all the cells, the priority of the cell LTE-F0 is highly set, and the mobile station UE is set such that it is easy to select the cell LTE-F0 as a camp-on cell.
A dual terminal compatible with the UMTS scheme and the LTE scheme such as a card-type mobile terminal that primarily performs a packet communication desirably resides in the cell within the radio access network of the LTE scheme; on the other hand, a dual terminal compatible with the UMTS scheme and the LTE scheme that primarily performs a voice communication desirably resides in the cell within a radio access network of a UMTS scheme.
This is because the LTE scheme is a mobile communication system dedicated to a packet communication and when the voice communication is concerned, it is probably used also in a mode that is offered in the UMTS scheme only. In consideration of a success rate for an origination and termination of a voice communication and the connection delay, the dual terminal that primarily performs a voice communication desirably camps on by preferentially selecting the cell within the radio access network of the UMTS scheme.
Therefore, in the broadcast information, the priority for the frequency used in the cell within the radio access network of the LTE scheme is highly set; however, it is considered that in the dual terminal compatible with the UMTS scheme and the LTE scheme that primarily performs a voice communication, the priority for the frequency used in the cell within a radio access network of the UMTS scheme is highly set by way of the dedicated signaling (RRC Connection Release message, etc.).
However, in such a case, the following problems occur. Such problems will be explained with reference to FIG. 5.
As illustrated in FIG. 5, the radio base station eNB notifies, to the mobile station UE (dual terminal compatible with the UMTS scheme and the LTE scheme) that is in communication in the radio access network of the LTE scheme, by way of the dedicated signaling, the priority “4” of the cell LTE-F0 and the priority “6” of the cell UTRA-F1, and instructs the mobile station UE to perform “Redirect” to the cell UTRA-F1.
As a result, in the mobile station UE, the priority “6” of the cell UTRA-F1 becomes higher than the priority “4” of the cell LTE-F0, and then, the mobile station UE starts camping on the cell UTRA-F1.
In step S2002, the mobile station UE performs the cell re-selection process based on the conventional radio quality between the cell UTRA-F1 and the cell UTRA-F2.
According to the present 3GPP specification, if the mobile station UE does not have any priority for a frequency different from a frequency used in a cell within a radio access network of the UMTS scheme when the mobile station UE resides in a cell within a radio access network of the UTRA scheme, then the mobile station UE usually performs the cell re-selection process based on the conventional radio quality.
In such an example, at the time of residing in the cell UTRA-F1, the mobile station UE does not have the priority for the frequency (frequency used in the cell UTRA-F2, for example) different from the frequency (carrier) used in the cell, within the radio access network of the UMTS scheme, and as a result, the mobile station UE performs the cell re-selection process based on the conventional radio quality.
When this results in the radio quality in the cell UTRA-F2 being superior to the radio quality in the cell UTRA-F1, the mobile station UE starts camping on the cell UTRA-F2 according to the cell re-selection process based on the conventional radio quality.
In step S2003, in the mobile station UE, if residing in the cell UTRA-F2 to which no priority is set according to the specification of the present 3GPP, then the lowest priority “−1” (priority smaller than the settable priority “0”) is automatically applied to the cell UTRA-F2.
In step S2004, the mobile station UE performs the above-described priority control between the cell LTE-F0 and the cell UTRA-F2. As a result, the priority “4” of the cell LTE-F0 is higher than the priority “−1” of the cell UTRA-F2, and thus, the mobile station UE starts camping on the cell LTE-F0 according to the above-described priority control.
Then, in step S2005, the priority “6” of the cell UTRA-F1 is higher than the priority “4” of the cell LTE-F0, and thus, the mobile station UE starts camping on the cell UTRA-F1.
Thereafter, in step S2006, again, the radio quality in the cell UTRA-F2 is superior to the radio quality in the cell UTRA-F1, and thus, the mobile station UE starts camping on the cell UTRA-F2.
Thus, a phenomenon in which the camp-on cell of the mobile station UE is repeatedly changed from the cell LTE-F0 to the cell UTRA-F1 to the cell UTRA-F2 to the cell LTE-F0 to . . . occurs.